Recent advances in microwave imaging have enabled commercial development of microwave imaging systems that are capable of generating two-dimensional and even three-dimensional microwave images of objects and other items of interest (e.g., human subjects). At present, there are several microwave imaging techniques available. For example, one technique uses an array of microwave detectors (hereinafter referred to as “antenna elements”) to capture either passive microwave radiation emitted by a target associated with the person or other object or reflected microwave radiation reflected from the target in response to active microwave illumination of the target. A two-dimensional or three-dimensional image of the person or other object is constructed by scanning the array of antenna elements with respect to the target's position and/or adjusting the frequency (or wavelength) of the microwave radiation being transmitted or detected.
Microwave imaging systems typically include transmit, receive and/or reflect antenna arrays for transmitting, receiving and/or reflecting microwave radiation to/from the object. Such antenna arrays can be constructed using traditional analog phased arrays or binary reflector arrays. In either case, the antenna array typically directs a beam of microwave radiation containing a number of individual microwave rays towards a point or area/volume in 3D space corresponding to a voxel or a plurality of voxels in an image of the object, referred to herein as a target. This is accomplished by programming each of the antenna elements in the array with a respective phase shift that allows the antenna element to modify the phase of a respective one of the microwave rays. The phase shift of each antenna element is selected to cause all of the individual microwave rays from each of the antenna elements to arrive at the target substantially in-phase. Examples of programmable antenna arrays are described in U.S. patent application Ser. Nos. 10/997,422, entitled “A Device for Reflecting Electromagnetic Radiation,” and U.S. patent application Ser. No. 10/997,583, entitled “Broadband Binary Phased Antenna.”
As a result, for each target, each antenna element is programmed with a particular phase shift to produce a beam of microwave radiation that experiences the highest constructive interference at the target. The combination of all the phase shifts assigned to the antenna elements in the array for a particular target is referred to as a pattern. The size of the pattern is the same size as the array, and each element in the pattern represents the phase shift of a corresponding antenna element in the array. In the case of a binary array, where each antenna element can introduce only one of two phase-shifts, the pattern can be represented as an array of ones and zeros.
To scan a person or other object, a number of patterns are typically pre-designed and stored for use in capturing successive microwave images of various targets associated with the person or other object. However, each pattern design is largely dictated by the frequency of the microwave radiation and the orientation of the antenna elements in the array to one or more of a microwave source (if applicable), microwave receiver (if applicable) and a particular target, and therefore, the patterns may not be optimal for various parameters of the microwave imaging system.
For example, background noise (often referred to as “clutter”) that results from stray radiation from the microwave source to the microwave receiver reduces the signal-to-noise ratio (SNR) of the microwave imaging system. Although a particular pattern may create the desired constructive interference at the target, that pattern may not be optimal to reduce the background noise at the microwave receiver. As another example, the phase changes between a pair of patterns used in scanning a person or other object may cause sidelobes that increase in area as the antenna phasing changes. In addition, as the number of antenna elements that require a change in phase between successive microwave images increases, there is a corresponding increase in the power necessary for scanning the person or other object. Therefore, what is needed is a flexible design technique for designing phase-shift patterns in microwave antenna arrays.